In the context of this specification, an “instream fluid power-extraction machine”, or simply “fluid power machine” or “fluid machine”, is any machine that is immersed or submerged in a stream of a flowing fluid, such as water or air for example, and that takes in or captures a portion of the flowing fluid from the stream, and that harnesses or extracts kinetic energy from the captured portion of flow to perform the primary intended function of the machine. For example, such a fluid machine may be a tidal hydropower turbine that harnesses energy from ebbing and flowing seawater to rotate a shaft thereby generating electricity for use at sea or on a nearby shore, or may be a turbine generator towed in fresh or salt water by a boat to produce electricity for use on the boat, or may be a windmill or wind turbine that harnesses wind power to produce electricity or provide mechanical shaft power for pumping water or driving other machines, or may be a tidal upweller that diverts flowing seawater upwards through a screen bed of seed clams or other shellfish seed to bring necessary nutrients to the seed stock, or may be any other machine that meets the criteria specified herein.
An instream fluid power-extraction machine, such as a hydropower generator, harnesses energy from a fluid stream that is not contained in a dam and is not constrained to necessarily flow through the fluid machine. The fluid machine can be stationary, as anchored in a stream flow, or moving through water as, for example, propelled by a boat. It can be of any size. One type of such fluid machine is an instream turbine that typically has one or more turbine rotors having vanes or blades configured for extracting energy from the stream in which it is immersed to power a rotating mechanism. It may provide electrical, hydraulic, or mechanical power for immediate use, for transmission to remote locations, or for storage.
In such turbine machines, various configurations of rotors and various orientations of the rotor axis with respect to the direction of ambient flow are known, as rotor axes may be oriented in-line with the flow of the stream, or vertically or horizontally transverse to the flow. Various configurations of flow feed into the rotors are also known, as rotors can be adapted to receive and redirect the intercepted flow axially, up or down, side to side, or radially inward or radially outward. In-line orientation of the rotor axis is exemplified in axial-flow propeller-type turbines used in water and air.
A radially inward fluid flow into a power-extracting turbine rotor, with a rotor axis transverse to the ambient fluid flow, is known in a turbine apparatus according to U.S. Pat. No. 8,506,244 and further developed by PfISH Turbines company of Machiasport, Me. An embodiment of such a turbine machine comprises two, matching, mirror-image rotors mounted on opposite ends of a rotatable shaft that is transverse to the stream, two spiral channels that surround and enclose the rotors and feed them with a spiraling inward flow of captured water, two intake funnels or a single common intake funnel that communicates with the two spiral channels and brings new flow in from the stream, and two spent flow channels that carry spent flow axially away from the ends of the rotors and discharge it back to the stream. The rotors take in flow all around their open intake peripheries, redirect it to an axial direction, and meanwhile harvest power from the flow. The entire disclosure of U.S. Pat. No. 8,506,244 is incorporated herein by reference.
All of such instream fluid power-extraction machines suffer several problems or limitations, however. A first problem is that the flowing fluid is not constrained to flow into the inlet(s) of the machine; instead the fluid can simply flow around the machine. A fundamental challenge of designing and operating an instream fluid machine is to get the fluid of the stream to flow into and through, and interact with, the machine rather than flowing around it. This contrasts markedly with conditions applying to a hydropower turbine that is fed water with an increased dynamic pressure and/or static pressure in a constrained manner from a dammed impoundment through a penstock, from which all of the water must flow through the turbine. On the other hand, in an instream fluid machine, the inlet, the entire internal flow path (including the flow conditions through the power-extracting rotor), and the discharge outlet cannot exhibit an excessive obstruction to the flow, because then to a corresponding extent the incident ambient flow will be diverted around the inlet(s).
A second problem is that extracting energy from the captured fluid (thereby producing “spent” fluid) necessarily reduces the spent fluid's total energy (and therewith its total pressure). A theoretical problem arises because if one could extract all of the kinetic energy from the flowing fluid, then the speed thereof would be reduced to zero, so that the spent fluid could not be expelled or discharged from the machine, and thus no new incident fluid could enter the machine and the energy extraction would fall to zero. Namely, the extracted power is proportional to new flow coming in. In view of this problem, the well known Betz limit specifies the maximum energy portion, about 59%, that can theoretically be extracted from the fluid flowing through such a turbine rotor.
A third problem is that the spent fluid must be discharged back into the ambient fluid stream that is flowing around the immersed machine, but the spent fluid has a lower total pressure and lower total energy, in comparison to the surrounding ambient flow because some of the captured fluid's energy has been extracted by the machine. The pressure prevailing at the discharge outlet(s) of the machine specifies the pressure at which the spent fluid must be discharged. If a diffuser, barrier or other structure is provided to reduce ambient pressure at the outlet, in an effort to “suck” more captured fluid through the machine, then such reduced pressure would just as much “suck” downstream ambient fluid backwards into the discharge outlet, thereby impeding the discharge of the spent fluid. Furthermore, the overall structure of such an instream machine acts as an obstruction of the ambient stream of fluid. Where smooth, e.g. laminar, ambient fluid flow encounters an obstruction, it forms a three-dimensional topology of disturbance around the obstruction, namely a shifting pattern of “separated flow” that varies in shape, location and intensity with the physical characteristics of the fluid (e.g. speed, density and viscosity) and the configuration, orientation and size of the obstruction in its path. Most importantly, this disturbance of the ambient flow produces vortices, turbulence, and varying lines of attachment and detachment in a “wake” downstream from the instream fluid machine, whereby the wake's turbulent zone persists for some distance downstream from the machine. Various wake and vortex patterns are commonly known. This downstream wake creates a “backwash”, which has little or no downstream component of flow, or even has an upstream component of flow, at the downstream side of the machine, and which can thus also hinder the discharge of spent fluid from the machine's discharge outlets. There is thus a longstanding problem and longfelt need in the art, to re-energize the spent fluid and rapidly discharge it back into the ambient flow.
Most existing conventional instream fluid power-extraction machines, such as tidal power turbines and wind turbines, do not include provisions to facilitate the reintroduction of the spent fluid flow back into the ambient stream at the discharge outlet(s) of the machine. Many such machines simply discharge the spent flow axially downstream back into the surrounding ambient stream, with no consideration given to attempting to efficiently introduce the lower-energy lower-total-pressure spent fluid into the higher-energy higher-total-pressure ambient stream. The result is a turbulent downstream flow with vortices and the like, due to the turbulent unguided or uncontrolled mixing of the spent flow with the ambient flow and due to the turbulence introduced by the structure of the machine itself, which can hinder the discharge and thus the through-flow of fluid through the machine.
Many designs of instream turbine are known. In principle of operation, they share a common characteristic, namely that their source of motive power is the pressure gradient that develops in the free flow field of ambient flow around the operating machine. If you make the fluid do external work you reduce its total pressure. The pressure difference between a turbine intake inlet that faces into the ambient stream and a turbine discharge outlet that is downstream drives through the machine the fluid that makes the machine perform its intended function. Regarding orientation of the machine, it is generally clear that an intake inlet should face directly at the oncoming flow, as velocity of the stream drives fluid toward and into the machine. Not so easily specified, however, is the orientation of a discharge outlet. The backflow inherent in the downstream wake opposes the flow from the discharge outlet.
The purpose of an instream fluid power-extraction machine such as a hydrokinetic turbine is to extract kinetic energy from the flowing stream in which it is deployed. At the inlet opening, entering flow carries the momentum of the stream. It is easy to see how captured internal flow expends energy as it drives the load, e.g. a rotor connected to a shaft driving an electric generator, but external flow expends energy as well. Consider internal and external flows meeting at the discharge outlet opening—pressure there is common to both flows. Failure to adequately re-energize the spent internal flow interferes with returning it to the stream. That reduces the internal volume flow rate or throughput, and thus the power, in comparison to what might be achieved.
Typically the discharge outlet of an instream turbine faces directly away from the stream, i.e. directly in the downstream direction. In some designs the discharge outlet is enlarged and moved rearward with a diffuser, also called an augmentation channel. Inside the diffuser, spent flow moving toward the discharge opening slows down, controlled by the expanding channel shape of the diffuser, while outside the diffuser the exterior flow that surrounds the machine is pushed outwards, away from the machine, by the outer surface of the diffuser. The diffuser allows the discharge outlet to be sized bigger than it otherwise would be. Also, these diffusers suffer an ever-diminishing effectiveness of the mixing and re-energization of spent flow as the machine becomes larger. Namely, the captured volume of fluid increases proportionally with the area of the intake inlet which increases proportionally with the radius squared, but the induction area of the diffuser increases proportionally with the circumference thereof which increases proportionally with the radius. Thus, the diffuser becomes less effective as the radius of the machine increases.
Slots are sometimes opened in the diffuser wall to let some of the exterior flow enter in and mix with the spent flow inside, attempting thereby to energize that spent flow and reduce the detractive effect of the so-called boundary layer near the inner surface of the diffuser, where friction slows the flow. Other designs take a different approach, having a second shroud outside of the first, whereby the second shroud captures some of the surrounding flow outside of the machine and redirects it inward and rearward to mix with the spent flow at the discharge outlet, to energize it, entrain it, and eventually carry it away. These approaches to discharge outlet design offer some benefit in turbine performance, but the backflow of the wake still works against the spent flow discharge, because the discharge outlet is still located in the downstream “flow shadow”, and thus in the turbulent wake and backflow, of the structure of the machine. Also, all of these known designs draw exterior ambient flow into the lower-pressure spent flow inside the discharge channel rather than directing interior spent flow out into the ambient stream. That has the effect of constraining the location of the mixing zone (where spent flow can be energized) to the “flow shadow” region of the ambient stream, downstream of the machine itself, thereby limiting the mixing zone as to its size and effect.
Instream hydrokinetic turbine machines having the above-discussed features such as a shroud around the turbine and an ejector structure around the shroud in order to draw exterior ambient water into the central axial discharge channel, are disclosed, for example in U.S. Pat. No. 3,986,787, U.S. Pat. No. 4,025,220, U.S. Pat. No. 7,832,979, U.S. Pat. No. 7,976,270, U.S. Pat. No. 7,980,811, U.S. Pat. No. 8,376,686, and U.S. Pat. No. 8,622,688. As mentioned above, all of those turbine machine designs merely attempt to induct exterior ambient water into the spent discharge flow that flows axially downstream from the downstream side of the rotor. There is no structure provided to prevent or avoid the influence of wake turbulence on the discharge and no structure provided to direct the spent flow by itself radially or laterally outwards into an accelerated ambient flow to bring about mixing and re-energization of the spent flow into an accelerated ambient flow outside of the machine structure. There is also no provision to make use of momentum of an accelerated exterior flow to bring about mixing of the spent flow with the exterior fluid outside of the machine.
In any instream fluid power-extraction machine such as a hydrokinetic turbine, the generated power is proportional to new flow coming in. Spent flow at the turbine discharge has equal volume as the new flow coming in, but less energy and speed. If the power of incoming flow were to be completely harvested, for example, then spent flow wouldn't even move as discussed above. Sluggish flow at the discharge is a fundamental problem. Discharge is additionally impeded by the backwash of the downstream wake. A basic question that remains to be answered is thus how the spent flow can be more effectively re-energized, entrained with the ambient flow, and rapidly carried away to make room for new flow entering an instream machine. Namely, how can you most efficiently and effectively energize the spent flow that comes from the machine, and reintroduce it back into the stream? The need for solving this problem is common to all fluid power-extraction machines. Momentum of the stream passing by provides the only energy available to use. The present invention addresses this problem with a novel approach.